FIGS. 1 and 2 of the accompanying drawings are schematic transverse sectional views of a prior art rotary vane pump and illustrate the arrangement of components in such a pump, and the way in which the pump operates. The rotor includes a rotor member 1 which is in this case cylindrical, and is mounted eccentrically within a cylindrical pumping chamber 2 formed in a pump casing 3. The casing 3 is formed with inlet and outlet ports 4 and 5 which open into the pumping chamber 2 at positions which are approximately diametrically opposed with respect to the axis of the rotor member 1. The rotor further includes a plurality (six in the illustrated arrangement) of circumferentially spaced vanes 6 which can slide inwardly and outwardly relative to the rotor member 1 in corresponding axially extending slots 7 formed in the rotor member 1. The slots in this particular case are non-radial, but are formed so that the vanes project forwardly by a predetermined angle .alpha. relative to the normal to the surface of the cylindrical rotor member 1. The slots could however be radial. In use, the vanes 6 are thrown outwardly relative to their slots by centrifugal force so that their outer edges are maintained in sweeping contact with the inner cylindrical surface 2' of the pumping chamber 2. The illustrated arrangement is adapted for clockwise rotation of the rotor.
As is well known, rotation of the rotor causes fluid displacement from the inlet 4 to the outlet 5 so as to create a higher pressure at the outlet than at the inlet. In FIG. 1, the rotor is shown in a position in which it has just trapped a "cell" of fluid A between the wall 2' of the pumping chamber, the outer cylindrical wall 1' of the rotor member 1, and two adjacent vanes 6. It will be seen that as the rotor rotates clockwise from this position, the cell A reduces in volume so as to compress the fluid, the maximum compression being attained when the cell A reaches the position shown in FIG. 2, i.e., when the vane at the leading end of the cell is about to reach the outlet port 5. Thereafter, the fluid is exhausted from the cell A into the outlet port. This process of cell creation, displacement and fluid exhaust occurs continuously at very high speed to produce the required pumping action.
The pump can generally be operated selectively in the compression or vacuum mode. In the compression mode, the equipment to receive the compressed fluid is coupled to the outlet port 5, the fluid being freely supplied to the inlet port. In the vacuum mode, the equipment to which the vacuum is to be applied is coupled to the inlet port, the fluid being freely exhausted from the outlet port.
Rotary vane pumps of this kind are generally reliable, efficient and rugged. However, when operating in the vacuum mode at very low absolute pressures, a large amount of heat is generated, and steps must be taken to remove this heat if overheating is to be avoided. It is known to form the outside of the casing 3 with cooling fins, but in general, cooling by natural convection over these fins is insufficient. In installations where a supply of cooling water is available, a sufficient degree of cooling can be ensured. However, in many applications water-cooling is not possible or at least inconvenient to arrange. As an alternative, it is known to employ air cooling using a fan arranged to blow cooling air over the outside of the casing. However, even this has disadvantages in that the cooling fan is noisy, and since it is normally driven from the pump shaft, imposes an additional load on the drive source.
To avoid these problems, it has been proposed in the past to reduce the effect of heat generation in the vacuum mode due to the compression of the fluid from the intake vacuum to a higher pressure, which is usually atmospheric, over the whole of the arc between the inlet and outlet ports. This has been done by introducing cooling fluid into the chamber through an additional port formed in the pump casing at a circumferential position which is upstream of the outlet port with respect to the direction of rotation of the rotor. With reference to FIG. 3 of the accompanying drawings, this additional port, hereinafter referred to as the ballast port, is indicated by reference numeral 17.
The circumferential spacing between the inlet port 4 and this ballast port 17 is slightly greater than the circumferential extent of an inter-vane cell, so that the inlet and ballast ports are never in communication. This is important, since the ballast port 17 is intended to relieve the vacuum in the cell by introducing ballast fluid at a high pressure relative to the vacuum into the cell. In the position of the rotor shown in FIG. 3, a cell A has just been closed, and the preceding cell B has almost completely swept past the ballast port 17, so that the vacuum in this cell B has been partly relieved by the introduction of ballast fluid.
The ballast port and the outlet will normally be open to the atmosphere, the inlet port being coupled to the equipment to which a vacuum is to be applied. It has been proposed to provide one or two such ballast ports.
Although the cooling effect achieved using existing arrangements for the introduction of ballast air has satisfactorily avoided the need for a cooling fan at low and medium vacuum levels, it has been found necessary to add a cooling fan if operation at high vacuum (in excess of 25 inches Hg gauge, or 635 mm Hg gauge) is required.